The waste water treatment is one of the main tasks of the human civilization. The continual production of sewage water makes in the U.S. alone for a streamflow (about 1000 m3/s) greater than that of the river Thames. The treatment aims at lowering the sewage volume, reducing its unpleasant odor, ridding it of hazardous chemicals, and at neutralizing microbial pathogens. The modern sewage treatment may comprise, beside separation techniques (such as sedimentation, filtration, centrifugation, or coagulation), also biological treatment (such as aerobic treatment or anaerobic digestion), heat treatment (such as pasteurization, drying, incineration), chemical treatment (such as using formaldehyde, sodium hydroxide, lime, strong oxidizers, etc.), and irradiation (such as using γ rays, UV light, electron beam). Before discharging the resulting sludge into the environment, e.g., to landfills or to the ocean or for agricultural use, its chemical and biological properties must comply with still stricter environmental standards.
The pathogenic factors, comprising viruses, bacteria, protozoa, or larger parasites such as worms, remain the most imminent health hazard presented by sewage water during all its processing stages. Some of the organisms, or their stable forms such as spores or cysts or eggs, may survive harsh conditions of various sewage treatments, and they may even get concentrated during dewatering the sewage into the sewage sludge. The microbial contamination of water is usually characterized by a coliform count, since the amount of Escherichia coli-like bacteria is widely used for indicating faecal pollution and assessing the risk of waterborn disease. The coliform count, being as high as 1011 CFU (colony forming units) in 100 g of raw sewage slurry, should be lowered to less than 105 or 102, depending on the country, type of regulation, and also according to whether the subsequent use of the resulting sludge is defined as unrestricted or restricted. For example, the Environmental Protection Agency (EPA) defines two types of biosolids in regard to pathogen reduction, Class B has a reduced level of pathogens, whereas Class A is nearly pathogen free.
Although there are various biocidal methods, to choose a cost-effective combination of available techniques is a non-trivial task in view of large volumes to be treated, many non-biological contaminants to be considered, and time-, place-, and volume-dependent costs of individual techniques. Therefore, there is a continuous need of new technologies for waste water treatment, and it is an object of the invention to provide a novel method for treating a raw sewage slurry.
Lime seems to have been used for deodorizing waste, or, as it is nowadays called, stabilizing it, from ancient times. The high pH, caused by calcium oxide or hydroxide, inhibits the microbiological activity and reduces the pathogens count. However, after subsequent acidification, e.g. by absorbing carbon dioxide, some pathogens may reappear. Another disinfection method uses the irradiation by high energy electrons. However, huge volumes of urban wastewater to be treated, together with high doses necessary for the complete pest annihilation make the method too expensive when used alone. Japanese Patent Publication No. 11319793 describes a method of decomposing organochlorine compounds in sludge by irradiating with an electron wave. Japanese Patent Publication No. 54109244 describes a method of treating organic sludge by adjusting its pH to 9-14 and subsequent irradiating it by electron beam. Korean Patent Application KR 208956 describes a method for treating wastewater by adjusting its pH, followed by the electron irradiation. However, the described methods either do not avoid the problem of great processed volumes resulting in costly, high energy demands for the irradiation stage, or said methods include additional treatments, such as biological treatments, making the whole process complex and expensive. U.S. Pat. No. 4,187,176 discloses a treatment plant for removing heavy metals from wastes by irradiating only that part of the sludge that contains heavy metals, which metals are converted into a well sedimenting form.
It is therefore an object of the invention to provide a method of treating sewage slurry comprising the irradiation with electrons, wherein said raw sewage slurry is first dewatered and thickened before said irradiating.
It is another object of this invention to provide a method of treating raw sewage slurry after dewatering and thickening, comprising irradiating with electrons followed by the treatment with a base.
It is also an object of this invention to provide a cost effective method for processing large volumes of urban wastewaters, which method includes partial dewatering of the raw slurry, irradiating obtained preconcentrated slurry with high energy electrons, and treating the irradiated sludge with a base, such as lime, thereby obtaining a final sludge for discharging into the environment, with allowable microbial count, possibly for use in soil recultivation or reclamation, and in agriculture.
Other objects and advantages of present invention will appear as description proceeds.